There is a strong existing need for flexible printed circuit (FPC) materials which have good electrical insulating properties, good mechanical properties, and high temperature dimensional stability to enable them to withstand soldering operations above 400.degree. F., preferably as high as 500.degree. F.
It is known in the prior art that various polyimide films can be adhesively bonded to copper foil to form such FPC articles. Nevertheless, the requirement of an adhesive layer adds an expensive and difficult process requirement. Moreover, the adhesive layer is the weakest link in the overall laminate. Therefore, such laminates suffer from important temperature limitations, which adversely affect significant characteristics, such as bond strength, dimensional stability, environmental stability, and chemical stability.
Another alternative taught by the art, see e.g., U.S. Pat. No. 3,682,960 is to use solutions of polyamic acids and/or amide-modified polyamic acids as intermediates which are soluble in certain solvents such as dimethylformamide. These solutions are then coated on various metal substrates such as copper sheets, copper foil, copper wire, etc. Subsequently, it is required that the coated substrate be exposed to the precisely proper temperature or free radical environment conditions necessary to achieve full cyclization without cross-linking or without inadequate cyclization.
Improved cyclized polymers adhering to the surface of a metal foil, which is to be used as a flexible printed circuit suffer from the adverse characteristic of lacking flexibility and foldability, which are the sine qua non of a flexible printed circuit.
The very existence of a flexible printed circuit market is strongly indicative of the market place demand of a printed circuit which is flexible enough to be stuffed, jammed, wrapped, poked into what would ordinarily be inaccessable areas. FPC's which have been prepared from improperly cyclized films will not stand up under these conditions, nearly as well as the FPC articles according to the present invention.